In the production of lime from limestone, the limestone is heated in a calcining chamber so as to decompose the same and drive off carbon dioxide according to the general equation: EQU CaCO.sub.3 +Heat.fwdarw. CaO+CO.sub.2
with heating of the limestone to a high temperature and holding it at that temperature for a period of time to effect the decomposition. This calcination, or "burning", requires a large amount of heat.
The limestone can be calcined in a shaft kiln or a rotary kiln. A rotary kiln generally comprises a long straight tubular steel shell that is lined on the inside with refractory brick. The tubular shell is usually inclined at an angle of about 3.degree. to 5.degree. to the horizontal and rests on trunions which rotate the tubular shell at about 1 to 2 revolutions per minute. A charge of limestone is fed into the upper end into the top of the kiln into a calcining chamber and slowly travels in a tumbling fashion through the length of the kiln. Heat is supplied into the lower end of the kiln through use of a burner that projects a flame into the calcining chamber between the charge passing through the calcining chamber and the refractory lining of the kiln. Oil, natural gas, or pulverized coal can be combusted in the kiln to provide the necessary heat, with heat transfer to the charge mainly achieved by radiation and conduction.
Because of the favorable price differential of coal over natural gas as a fuel, many calcining kilns in the United States have been switched from natural gas to pulverized coal firing. The problems associated with such a switch involve not only the additional capital costs and maintenance costs of coal handling and pulverizing equipment, but the fact that the molten slag, or ash, from the coal leaving the flame front deposits on the walls of the calcining chamber of the kiln and builds up a hard glassy fused-slag ring about the wall of the calcining chamber. This build-up eventually plugs the kiln, and has to be periodically removed. In some installations, kiln production is being lost for periods of several days, every ninety days of operation.
The slag ring problem is theoretically manageable. Calcining temperatures, in the vicinity of 1700.degree. F., are below the ash fusion temperatures of most United States coals. If, in fact, calcining proceeded at or slightly below the minimum ash fusion temperatures the fly ash from the coal would not become a liquid slag and therefore would not stick to the kiln walls. However because of the poor heat transfer characteristics of the non-uniform limestone bed, the kilns are overfired and temperatures are permitted to rise far above the molten slag forming temperature. The resultant slag ring problem is evidently considered preferable to the incomplete calcining that would result from lower firing rates using conventional kiln lengths.
More vigorous mixing of the limestone bed would expose more of the material to the radiant flame front and would therefore allow a reduction in firing rate for the same calcining efficiency. However this would also result in the creation of more lime fines which is the major air pollutant from the kilns.
It is an object of the present invention to provide an improved lime kiln that is designed to retard the formation of a slag ring on the walls of the calcining chamber when coal is combusted to provide heat for calcination of lime passing through the calcining chamber.
It is another object of the present invention to provide a method for retarding the formation of a slag ring on the inner wall of a rotary lime kiln, during calcination of lime in a calcining chamber of the kiln.